In connecting conventional thermoplastic resin pipes made of polyethylene (PE) and polypropylene (PP) used for a gas pipe, a water pipe or other purposes, thermal fusion methods has widely been used to obtain perfect air tightness.
Conventional thermal fusion methods include butt fusion welding, heat fusion socket welding, and electrofusion socket welding or electric fusion.
Among these methods, electrofusion socket welding (hereinafter referred to as ‘electrofusion’) provides excellent fusion quality and thus is used for a pipe such as a gas pipe where air-tightness is required. Electrofusion employs an integrated type of socket prepared by injection molding performed by inserting a heat element such as a heating coil into an inside of a coupling socket. A heating coil employs a metal wire having an electric resistance which is maintained to some extent, such as a copper wire, a copper alloy wire or a nichrome wire. For example, an resistive wire or spiral shaped resistive wire coated with a thermoplastic resin is wound to have a coil shape and an electric terminal is connected at the end of the coil. Then, an electric resistance wire-integrated coupling socket is prepared by inserting the resulting product into an injection mold via injection molding.
Generally, a material which is the same with a material of a pipe is used as a material for an electrofusion coupling pipe. For example, the material used for a coupling pipe used for connecting a PP pipe is PP.
An electrofusion socket prepared in this manner is fit to a pipe to be fused and then electric power supplies to the electrofusion socket for a certain period of time. As a result, the pipe is connected to the electrofusion socket as the external diameter side of the pipe and the internal diameter side of the electrofusion socket are simultaneously melted.
Such an electrofusion method is easy to perform and provides excellent fusion quality, and thus the electrofusion method is mainly used for a pipe such as a gas pipe where air-tightness is required. However, since the manufacturing process of an electrofusion socket is complex, the price of an electrofusion socket is high, and thus an electrofusion socket is not commonly used as a general piping material for civil-engineering/construction work.
In addition, since a metal heating coil has a low resistance and thus requires a high supply current, a heavy power supply is necessary. For example, when an electric resistance of a heating coil is 1Ω and an electric voltage of 40 V is applied, 1600 W of electric current is supplied. Therefore, as a voltage converter for supplying a low voltage and a thick power line for supplying a high current are required, a general electrofusion power supply weighs about 20 kg, portability thereof is extremely low.
In addition, when the temperature of a synthetic resin increases during the electrofusion work, the resin may gain flowability and thus heating coils arranged in a constant interval may be moved and adhere to each other. As a result, local heating may occur and the electrofusion quality may be decreased.
In addition, in conventional electrofusion coupling methods, the size of a coupling pipe was recognized by attaching a bar code to a coupling pipe or a comparable resistance terminal was inserted to an inside of a coupling pipe. These size recognition methods are additional factors which increase the product unit price. In addition, as the outdoor temperature is decreased, the energy required for coupling pipe electrofusion increases. Thus, supplied energy needs to be varied depending on the seasons. However, since conventional electrofusion coupling methods do not have a function of controlling supplied energy depending on the outdoor temperature, a constant current at which over electrofusion takes place to some extent is generally applied. Therefore, when the outdoor temperature is lower than zero degree Celsius, electrofusion may not be performed.
While a gas pipe which is generally used for electrofusion coupling is made of PE, a PP pipe is used for water supply and hot water supply. In comparison with a PE resin, a PP resin has a higher melting temperature (164° C. of PP resin vs. 135° C. of PE resin), a lower combustion temperature (about 300° C. vs. about 410° C.), and a low heat conductivity (0.15 W/mK vs. 0.4 W/mK). Therefore, when the same power (voltage) as that of the PE pipe is applied to the PP pipe, heat transport to the PP pipe is slower and thus a PP resin around a metal coil of a coupling pipe starts to be melt even before an outer diameter side of the PP pipe is melted. In other words, for connecting a PP pipe by a conventional electrofusion coupling method, a low power (low voltage) should be supplied for a long period of time and thereby time for electrofusion increases.
Patents PCT/KR2007/000540 and PCT/KR2009/003620 relate to a method of using a conductive polymer composite as a cylinder type heating element instead of a heating coil, which has been used in conventional electrofusion methods. A matrix polymer of a conductive polymer composite is usually the same as the material of a pipe which is to be fused.
However, in a case where a conductive polymer composite is used as a heating element, when a constant voltage is supplied for a predetermined time for each pipe size, the resistance of the heating element increases and a low power is supplied in the early stage of electrofusion. However, after the heating element has melted, the resistance is drastically decreased and thus a high power is applied. Then, the temperature of the heating element is drastically increased and thereby the heating element and the polymer around the heating element burn out, resulting in unstable fusion of the pipe and the coupling pipe. When the voltage is lowered to perform stable electrofusion, the time for electrofusion becomes longer.